Metabolic augmentation of human locomotion has proved an elusive goal. Although a number of exoskeletons have been built, none has demonstrated a significant reduction in metabolic demand of locomotion. Exoskeletons may loosely be classified as intended to augment human capabilities, such as load capacity or ambulatory speed, or to increase human endurance, by lowering the metabolic demand of the given activity. For example, an exoskeletal device intended to reduce the metabolic demand of movement may alternatively permit the execution of that movement at higher speed for a given metabolic demand. Other devices, intended to restore lost functionality, may also be thought of as exoskeletons.
Exoskeletons are classified as passive, quasi-passive or active, based on the usage of power. Passive exoskeletons require no energy source and generally consist of linkages, springs, and dampers. They typically rely on mechanisms, are less robust and, consequently, may result in behavior that may lead or lag what is intended. Active devices, in contrast, add energy to the gait cycle, usually through motors or hydraulic cylinders. Active systems are often limited by weight limitations necessary to minimize changes in momentum that occur during gait cycles, particularly during running. Quasi-passive devices lie between passive and active devices, being unable to inject energy into the gait cycle, but nonetheless requiring a power supply, usually to operate electronic control systems, clutches or variable dampers. Typically, although not necessarily, the power requirements of a quasi-passive device are relatively low. Further, exoskeletons, whether active, passive or quasi-passive, may be described as primarily acting in series or in parallel with a subject's limbs.
Moreover, the mechanics of walking and running are significantly different. Specifically, walking resembles, and can be modeled as, an inverted pendulum wherein kinetic and gravitational potential energies are substantially out of phase. During running, however, kinetic and gravitational potential energies are almost perfectly in phase, whereby the center of mass and, thus, potential energy are highest at essentially the same time as each other. In other words, during running, elastic potential energy is stored in muscle-tendon units in a cycle that is out of phase with kinetic and gravitational potential energy. Generally, active, passive and quasi-passive exoskeletons do not accommodate the running gait of a legged animal, such as a mammal, including, for example, a human wherein the center of mass and, thus, potential energy is highest at approximately the same time velocity and, thus, kinetic energy is highest (in phase), and, whereby elastic potential energy must be stored out of phase with kinetic and gravitational potential energy.
Therefore, a need exists for an exoskeleton that can augment running in a mammal, such as a human, that overcomes or minimizes the above-referenced difficulties.